1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus, and more particularly to an image forming method and an image forming apparatus for forming an image on a recording medium by using an ink.
2. Description of the Related Art
In general, an inkjet recording apparatus comprises an inkjet head in which a plurality of nozzles are formed, and forms an image on a recording medium by ejecting ink droplets respectively from the nozzles onto a recording medium; such apparatuses are used widely due to their excellent low-noise operation, low running costs, and their ability to record images of high quality onto recording media of many different types.
Japanese Patent Application Publication No. 6-184478 discloses a method in which the adhesiveness between a recording medium and an ink image is improved by printing ink onto a recording medium and fixing the ink by applying heat and pressure.
Furthermore, Japanese Patent Application Publication No. 2007-196417 discloses a single-pass printing method for printing by ejecting droplets of ink from an ink head onto normal paper using an ink containing water, water-soluble organic solvents and a pigment, the water content ratio being equal to or greater than 10 wt % and less than 50 wt % of the total weight of ink, the SP value of the water-soluble organic solvent contained in highest ratio of the water-soluble organic solvents being equal to or greater than 16.5 and less than 24.6, and the content ratio of the water-soluble organic solvent having this SP value being equal to or greater than 30 wt % of the total weight of ink, wherein the normal paper is pressurized after ejecting the droplets of ink, thereby fixing the ink.
However, in the method described in Japanese Patent Application Publication No. 6-184478, when performing high-speed recording using a water-based ink, since the fixing process is carried out before the drying of the ink has progressed sufficiently, then drying is inadequate and the image is conveyed to the fixing unit while still containing a large amount of water. If it is sought to fix the image by directly contacting the printed surface of the recording medium with a heating roller, then there are possibilities that the image flows due to insufficient aggregating force in the image, and image offset onto the heating roller occurs. This offset arises because of insufficient strength of the image with respect to the tensile forces acting in the upward and downward directions during the shearing and separating actions applied to the image in the fixing nip operation.
Japanese Patent Application Publication No. 2007-196417 makes it possible to suppress curling and cockling by setting the water content in the ink to less than 50 wt % and using a high content ratio (30 wt % or greater) of water-soluble organic solvent, but there are possibilities that a large amount of organic solvent remains in the image, giving rise to offset of the image onto the heating roller and a decline in fixing properties. Furthermore, as described in Japanese Patent Application Publication No. 2007-196417, problems also arise when pressure is applied to normal paper, but when recording onto a recording medium having a slow rate of permeation of the liquid component, such as a coated paper, the water content is liable to remain on the paper and the problems described above become critical.